Preparation of Alkenes:Elimination Reactions A.Dehydration of alcohols 1. Zaitsev rule:regioselectivity and stereoselectivity -the major product from B-elimination is the more stable alkene △ regioselective OH 90% 10% H3PO4, major minor HS04 OH 56% 32% 12% stereoselective
A. Dehydration of alcohols 1. Zaitsev rule: regioselectivity and stereoselectivity -the major product from -elimination is the more stable alkene OH + H 2SO 4 90% 10% OH H 3PO 4 + OH + + H 2SO 4 56% 32% 12% major minor regioselective stereoselective Preparation of Alkenes: Elimination Reactions
2.the acid-catalyzed E1 mechanism (EIcA) Lower energy product has lower Both products Ea,is formed come from same faster. intermediate
2. the acid-catalyzed E1 mechanism (E1CA ) R Both products come from same intermediate. Lower energy product has lower Ea , is formed faster
Give the expected major product for each reaction. Check your answers. H2S04 OH H3PO4 OH Both major products are the more highly substituted alkenes,which are the more thermodynamically stable
Give the expected major product for each reaction. Check your answers. OH H 2SO 4 OH H 3PO 4 Both major products are the more highly substituted alkenes, which are the more thermodynamically stable
B.Dehydrohalogenation of alkyl halides BH+X⊙ Elimination strong base:KOH/ethanol 8, CHCH,ONa/CHCH,OH EtONa_ tBuOK/tBuOH EtOH KOH EtOH tBuOK BuOH preferred for 1°RX
B. Dehydrohalogenation of alkyl halides C H C X + B C C + BH + X strong base: KOH/ethanol CH3 CH2 ONa/CH3 CH2 OH tBuOK/ tBuOH Br EtONa EtOH Cl KOH EtOH Cl tBuOK tBuOH preferred for 1º RX Elimination
Some Bulky Bases CH3 CH(CH3)2 H3C-C-0 :N-CH(CH3)2 CH3 H H3C CH3 tert-butoxide diisopropylamine 2,6-dimethylpyridine (CHCH2)N triethylamine
Some Bulky Bases C CH 3 H 3 C CH 3 O _ tert-butoxide (CH3 CH2 )3 N : triethylamine N H CH(CH 3 ) 2 CH(CH 3 ) 2 diisopropylamine H N CH 3 3 C 2,6-dimethylpyridine